1. Field of the Invention
The present invention relates generally to chain jacks and more particularly to chain jacks utilizing logic control.
2. Prior Art
Chain jacks are used often in marine ship building and construction operations to lift heavy loads precisely and economically. They generally comprise a hydraulic cylinder or base having a piston or ram attached to a head. Moveable latches are attached to, or made as part of, the base and head, each of which engages a chain between links thereof. Actuation of these latches is controlled such that they move alternately in and out of engagement with the chain links as the ram moves in and out of the hydraulic cylinder. The load is sequentially supported by one latch and then the other. Conveniently, the hydraulic cylinder may extend the ram at each stroke at least nominally equivalent to one pitch of the chain. The latch in the ram head will be engaged with the chain during piston extension while the base latch will be released. When the piston has reached the end of its stroke, the base latch engages the chain and the ram head latch is released while the ram contracts. Once contracted, the ram head latch engages the chain and the base latch is released again. The cycle is repeated advancing the chain another short distance.
Such prior art chain jack systems suffer the following limitations. The latch activation is slow due to hydraulic head loss between the control panel and the jack latch mechanism. This results in slower speeds of operation for the jack system. Individual jacks cannot be operated out of synchronization because of the complex latch control operations required when operating several jacks which are not synchronized. Manual control of activation of the latch mechanisms require special precautions of the retract cycle of the chain jacks during that period of time in which the latch mechanisms are extended and retracted prior to the chain jack piston reaching its maximum and minimum limits of travel. In addition, a minimum of four hydraulic lines are required between the powered control console and the jack for latch control functions.
Since chain jacks are most often employed for handling heavy loads, construction of their parts and components must be correspondingly heavy. The latches employed must be strong enough to support the load carried by the jack. To obtain this strength, the thickness of the latches may take up almost all the available space or window between adjacent chain links lying in the same plane. Therefore, very close control of latch actuation is necessary so that each latch enters into engagement and is released from the chain at exactly the proper position of the chain with respect to the cylinder or base and the head of the ram. The present invention provides latch actuation control synchronized with the ram head movement such that one pair of latches cannot be disengaged unless the other pair is supporting the load. In addition, the present invention provides precise control of latch entry into each window between links even when this window is only a small amount wider than the thickness of the latch.